estimate absolute timestamp more precisely (#5078)

When the absolute timestamp of incoming frames was not available, it
was filled with the current timestamp, which is influenced by latency
over time.

This mechanism is replaced by an algorithm that detects when latency is
the lowest, stores the current timestamp and uses it as reference
throughout the rest of the stream.

internal/staticsources/mpegts/source.go

@@ -120,6 +120,7 @@ func (s *Source) runReader(nc net.Conn) error {
 	res := s.Parent.SetReady(defs.PathSourceStaticSetReadyReq{
 		Desc:               &description.Session{Medias: medias},
 		GenerateRTPPackets: true,
+		FillNTP:            true,
 	})
 	if res.Err != nil {
 		return res.Err